1. The change from the low intensity mobbing call 'duck' to the high intensity call 'tix' involves the addition of what is probably the first overtone and a slight rise in pitch of the fundamental tone. The first change has a lower threshold than the second. 2. Bouts of calls are usually associated with flights and commonly begin a little before take-off. 3. At or near take-off there is a change (the 'flight effect') which tends to initiate calling if it has not begun, and to accelerate it and/or cause the change from 'duck' to 'tix', if it has. 4. Experimental prolongation of flight prolongs calling and promotes the change to a 'tix' type of call. 5. Bouts of calls given when perched are accompanied by activity of the system controlling flight, which does not reach the threshold for take-off.